Method of producing alkali metal chlorate

ABSTRACT

VOIR CONTAINING THE ALKALI METAL CHLORATE PRODUCED IN THE MIXING ZONE.   ALKALI METAL CHLORATES ARE PRODUCE BY PROVIDING A MIXING ZONE FOR THE REACTION OF A CHLORINE CONTAINING ANOLYTE AND ALKALI METAL HYDROXIDE CONTAINING CATHOLYTE FORMED BY THE ELECTROLYSIS OF AN ALKALI METAL CHLORIDE IN A DIAPHRAGM TYPE ELECTROLYTIC CELL IN SUCH MANNER THAT SEPARATE ANOLYTE AND CATHOLYTE FEED SOLUTIONS ARE INTRODUCED INTO THE RESPECTIVE COMPARTMENTS OF THE ELECTROLYTIC CELL. THE CATHOLYTE FEED SLOUTION IS DRAWN FROM A RESER-

Jan. 5, 1971 M. P. GROTHEER ETAL METHOD OF PRODUCING ALKALI METAL CHLORATE Original Filed Nov. 30, 1965 4 Sheets-Sheet l Jan. 5, 1 971 p GRGTHEER ET AL 3,553,088

METHOD OF PRODUCING ALKALI METAL CHLOHATE Original Filed Nov 30, 1965 4 Sheets-Sheet 2 Jan. 5, 1971 M. P. GROTHEER ET AL 3,553,038

METHOD OF PRODUCING ALKALI METAL CHLORATE 4 Sheets-Sheet 3 Original Filed Nov. 30, 1965 Jan. 5, 1971 M p GROTHEER ET AL 3,553,088

METHOD OF PRODUCING ALKALI METAL CHLORATE Original F iled Nov. 30. 1965 4 Sheets-Sheet 4 O (O m u] m u H E H a I! g 3 I I U U N m 1 LL! E n: J O a g a- S? w, m J g *9 3 m m P u q E N g] m o w a: E n

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nitecl States Patent 3,553,088 METHOD OF PRODUCING ALKALI METAL CHLORATE Morris P. Grotheer, John E. Currey, and Edward H.

Cook, Jr., Lewiston, N.Y., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Original application Nov. 30, 1965, Ser. No. 510,617.

Divided and this application May 27, 1969, Ser.

Int. Cl. Btllk 1/00; C01b 11/26 US. Cl. 204-95 8 Claims ABSTRACT OF THE DISCLOSURE Alkali metal chlorates are produced by providing a mixing zone for the reaction of a chlorine containing anolyte and alkali metal hydroxide containing catholyte formed by the electrolysis of an alkali metal chloride in a diaphragm type electrolytic cell in such manner that separate anolyte and catholyte feed solutions are introduced into the respective compartments of the electrolytic cell. The catholyte feed solution is drawn from a reservoir containing the alkali metal chlorate produced in the mixing zone.

This application is a division of application S.N. 510,617 filed Nov. 30, 1965.

This invention relates to the production of alkali metal chlorates and to a novel electrolytic cell therefor. More particularly, the present invention relates to a method for producing alkali metal chlorates in a novel diaphragm type electrolytic cell which provides greatly improved anode current efficiencies and improved over-all cell efiiciency.

Previously, alkali metal chlorates were produced commercially by electrolyzing brine solutions such as a sodium chloride solution between anode and cathode electrodes. Such cells operated at a maximum current elficiency of about 90 percent. Current efficiencies increased with increasing current densities within the range of about 0.6 to 1.0 ampere per square inch, but decreased with increasing current densities above about 1.0 to 1.2 amperes per square inch. Thus, it was preferable to operate at the lower current densities using an electrolyte having a pH of about 6.5 to 7.5 Under such conditions, chloride ions oxidized at the anode to produce atomic chlorine which reacted immediately with water or caustic to form hypochlorite ions. The hypochlorite ions were formed on or extremely close to the surface of the anode. Being anions, they were attracted to the anode and many of them reacted to produce chlorate electrochemically.

The most desirable process to obtain the highest current efiiciencies involves electrochemical production of hypochlorit and chemical production of chlorate from the hypochlorite. Thus, both electrochemical and chemical reactions are involved in producing chlorate. If all of the chlorate is produced from hypochlorite electrochemically, the maximum current efiiciency of the cell would be about 67 percent. When the current efliciency of a chlorate cell is 90 percent, about 30 percent of the chlorate is being produced electrochemically and about 70 percent chemically. Because at least a portion of the chlorate was formed from hypochlorite electrochemically, rather than chemically, previous processes were inherently limited to a low maximum current efiiciency.

Thus, it is desirable to provide a chlorate cell which is suitable for operation at higher current densities and in which the electrochemical production of chlorate from hypochlorite can be substantially reduced or eliminated.

It is an object of the present invention to provide an electrolytic cell for the production of chlorate having greatly improved current efliciencies. Another object of the present invention is to provide a chlorate cell particularly suited for operation at higher current densities. It is a further object of this invention to provide a continuous process for the production of chlorates by means of an improved electrolytic chlorate cell. These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

In accordance with the invention, there is provided an electrolytic cell for the production of an alkali metal chlorate comprising a container for liquid having an anode and a cathode separated by means of a porous diaphragm to form an anolyte compartment and a catholyte compartment, means for feeding electrolyte to said anolyte compartment, and separate means for feeding electrolyte to said catholyte compartment with said anolyte and catholyte compartments being in communication with a mixing chamber for effluent gases and liquors from said anolyte and catholyte compartments. The present invention further provides a method for the production of an alkali metal chlorate in an electrolytic cell having an anode and a cathode separated by a porous diaphragm to form an anolyte and a catholyte compartment compris ing imposing a decomposition voltage between said electrodes, feeding an acidified solution of alkali metal chlo ride to the anolyte compartment, feeding a separate alkali metal chloride containing solution to the catholyte compartment, producing chlorine at said anode and alkali metal hydroxide at said cathode, passing said chlorine and said alkali metal hydroxide to a mixing zone and mixing and reacting said chlorine and alkali metal hydroxide thereby producing hypochlorite and subsequently an alkali metal chlorate.

The present invention provides a much more efficient process for producing chlorates by providing a means for eliminating electrochemical production of chlorate from hypochlorite, thus eliminating needless expenditures of current. In a further embodiment of the inven tion, a recycle process is provided whereby the chlorates produced by the cell are continuously or periodically removed by crystallization and wherein the mother liquor is returned to the cell with replenishing amounts of alkali metal chloride for further electrolysis. A particularly desirable feature of the present invention is that conventional chlorate cells can be modified in accordance with the present invention to provide more efiicient chlorate production.

The invention will be further described with reference to the drawings in which:

FIG. 1 is a vertical sectional view illustrating a cell of the present invention submerged in an electrolyte reservoir;

FIG. 2 is a horizontal sectional view of FIG. 1 along 2-2 further illustrating the cell of the present invention;

FIG. 3 is an enlarged vertical sectional view of a part of the cell of FIG. 1, further showing the anolyte and catholyte compartment detail thereof; and

FIG. 4 is a flow sheet illustrating a continuous process for the production of alkali metal chlorates by the present invention.

The present invention comprises an electrolytic cell 10 which is conveniently enclosed in an electrolyte reservoir 12 which is normally filled with electrolyte 14 during the operation of the cell. Thus, electrolytic cell 10 can be submerged in electrolyte 14 to a desired level as provided for by support 16.

The electrolytic cell 10 comprises an anode 18, or series of anodes, a cathode 20, or series of cathodes, an anolyte compartment 22, a porous diaphragm 24 and a catholyte compartment 26. Above the anolyte compartment 22 and catholyte compartment 26 is a mixing chamber 28. The illustrated cell is a bipolar type cell and, therefore, separators 30 are preferably provided to reduce leakage of electric current around the electrodes. Anolyte compartment feed means are provided by means of a manifold 32 and feed line 33. Th catholyte compartment electrolyte feed means 34 open to electrolyte reservoir 12 and provide means for intake of electrolyte into the catholyte chamber. Mixing chamber 28 is provided with exhaust port 36 for the passage of liquid efliuent out of the mixing chamber into electrolytic reservoir 12.

The anolyte compartment is preferably provided with a lid 38, having an opening 40 for the controlled passage of fluids from the anolyte compartment 22 into mixing chamber 28. In the operation of the cell 10 electrolyte is fed by means of pump 42 and line 44 to manifold 32 which feeds anolyte compartment 22 by means of feed line 33. As a decomposition voltage is passed through the cell, the reaction at the cathode in producing gaseous hydrogen causes pumping action of electrolyte 14 from electrolyte reservoir 12 through catholyte compartment feed means 34 into catholyte compartment 26 and subsequently to mixing chamber 28. The feed to the anolyte compartment is at a controlled variable rate determined by pump 42 and the capacity of manifold 32 and feed line 33. Anolyte liquor flowing through the anolyte compartment is electrolyzed therein to form chlorine, which escapes therefrom as a gas. The chlorine produced at the anode is passed through opening 40 into mixing chamber 28 wherein it is reacted with the hydroxyl ions produced in the catholyte compartment.

Mixing chamber 28 is preferably of sufiicient depth so that sufficient residual time is given to the chlorine gas in passing through the chamber to be entirely absorbed. The preferred depth varies with the temperature, which affects the reaction rate, the pH in the mixing chamber and the rate at which chlorine is being produced. Higher current densities produce chlorine and caustic at a faster rate and therefore, the current density usedis also a factor to consider. The depth of the mixing chamber can therefore vary considerably with the particular design and mixing action provided. The best depth can readily be ascertained by those skilled in the art. As a general indication, the mixing chamber preferably has a depth of about /2 to 2 times the height of the electrodes. In mixing and reacting chlorine and hydroxyl ions in mixing chamber 28, hypochlorite is formed which rapidly converts to chlorate under the preferred reaction temperatures and conditions.

The reactants are passed from mixing chamber 28 by means of exhaust ports 36 into electrolyte reservoir 12. Electrolyte reservoir 12 is of suflicient capacity to provide a limited residence time prior to the electrolyte being recirculated to the catholyte chamber. As such, it is preferably of a larger liquid capacity than the cell 10. In electrolyte reservoir 12, residual hypochlorite is provided with an aging time under conditions which favor chemical conversion to chlorate. Thus, in providing an aging period, electrolyte 14 normally contains less than about one percent hypochlorite as it is recirculated to the catholyte compartment 26 of cell 10 for further reaction.

As is further illustrated in the flow sheet in FIG. 4, the concentration of chlorate increases in the electrolyte 14 in electrolytic reservoir 12. This electrolyte is withdrawn by means of pump 46 from chlorate cell reservoir 47 and passes to crystallizer 48. Chlorate is separated in crystallizer 48 by conventional crystallization techniques, coupled with filtration, centrifuging or other liquid-solid separation techniques to produce solid chlorate 50 and mother liquor 52. The mother liquor 52 contains alkali metal chloride and residual amounts of chlorate which can vary in amount from about 10 to about 750 or more grams per liter for NaClO depending on the crystallization conditions used. This liquor can be returned to electrolyte reservoir 12 or preferably resaturated With replenishing amounts of alkali metal chloride 53 in saturator 54 prior to returning to the anolyte compartment of the electrolytic cell. In the feed line to the anolyte compartment of the electrolytic cell, the pH of the feed solution is preferably controlled by the addition of HCl 56 or chlorine to provide an acidic feed brine for the anolyte compartment so that the pH in the anolyte compartment will be in the range of about one to four.

The process of the present invention is suitable for producing alkali metal chlorates such as sodium chlorate, potassium chlorate, lithium chlorate, rubidium chlorate, cesium chlorate, and the like. However, because of the ready availability and the favorable solubilities, sodium chlorate is the normally produced chlorate from which numerous other chlorates are formed. Since sodium chlorate is the most commonly produced chlorate the invention will be further described with particular reference to sodium chlorate. However, in describing sodium chlorate, it is to be noted that other chlorates are produced in the same manner by the present invention.

Although the drawing illustrates a bipolar electrolytic cell having graphite electrodes, the present invention is equally applicable in the same manner to other cells using monopolar electrodes. The type of electrodes can also be varied substantially. Various metals, as well as carbon electrodes can be used wherein the cathode can be any metal including steel, copper, nickel, stainless steel, platinum, and the like materials commonly used as cathodes and wherein the anode is a more resistant material such as platinum, platinum-coated titanium or tantalum, lead dioxide, magnetite, or graphite.

The diaphragm used is of any suitable organic or inorganic material which is resistant to the environment within the cell and which can be fabricated into a porous barrier. Materials such as asbestos, Teflon, after chlorinated polyvinyl chloride, polyvinylidene chloride, and the like, are particularly suitable. The diaphragm can be made by weaving the desired material into a cloth which can be inserted between the electrodes using a frame or other means of support.

In the present invention, separate feeds are provided for the anolyte compartment and the catholyte compartment. The anolyte feed material is a saturated or nearly saturated solution of sodium chloride acidified with HCl or chlorine which may also contain up to about 75 0 grams per liter of sodium chlorate. Preferably, the anolyte liquor contains at least grams per liter of NaCl which may be the saturating amount at the operating temperature and concentration of NaClO More preferably, the NaCl concentration is to 300 grams per liter and the NaClO concentration is zero to 600 grams per liter. Preferably, the pH is controlled so that the electrolyte in the anolyte compartment has a pH in the range of about one to four and more preferably a pH of about three. The anolyte feed rate is controlled so as to be equal to, or more preferably, less than the catholyte feed rate. Thus, the anolyte feed rate is equal to about 5 to 100 percent that of the catholyte.

The catholyte feed is the same electrolyte as is contained in the electrolyte reservoir surrounding the cell. Alternatively, this electrolyte feed can come from a storage vessel apart from the cell since the reservoir need not surround the cell butas such, it is most convenient. The catholyte feed normally comprises about 50 up to about 750 grams per liter sodium chlorate and a nearly saturating amount of NaCl at the NaClO concentration. The NaCl content is preferably at least about 100 grams per liter and more preferably more than about 130 grams per liter. The sodium hypochlorite concentration is preferably less than about five grams per liter. The pH of this feed liquor is that of the reservoir, which is preferably about seven. The most preferred catholyte feed 'material is an aqueous solution containing about 100 to 500 grams per liter of sodium chlorate, about 100* to 250 grams per liter sodium chloride, depending on the NaClO concentration, and as low a hypochlorite content as is readily attainable, i.e., less than about 0.5 gram per liter. The electrolyte feed rate to the catholyte compartment is normally a natural feed rate generated by the electrolysis taking place in the cell. The rapid evoluchambers between each pair of electrodes. The glass sheets extended upward above the electrodes to form separators in the mixing chambers above the electrodes. The diaphragm separates the electrodes and forms separate anolyte compartments and catholyte compartments ttoh of hydrogen gas at the cathode Causes a rapid intake to which separate electrolyte solutions are fed at, or near of electrolyte into the catholyte compartment. Alterh bottom of h compartment hately, P p g means can he Provided to increase the The anolyte feed solution used in operating the present natural fiOW rate- However, the natural flow fate is cell in accordance with the invention was either an acidiy q fied sodium chloride solution or an acidified sodium The Cell Operating temperature is Preferably in the chloride solution containing varying amounts of sodium range of about 40 to 100 degrees Centigrade and more chlorate as indicated in the various examples. The anolyte Preferably about 60 to 90 degrees Centigrade The P feed containing sodium chlorate was representative of a tel'fed higher temperatures are Preferably used With continuous process as shown in FIG. 4 wherein mother creasing concentration of chlorate in the feed solutions. liquor is returned to h anolyte Compartment f h ll Increased temperatures reduce the electrolyte resistance With a replenishing amount f di hl id while further promoting the conversion of hypochlorite A Ihixinh chamber provided above h 11 h d an to ehibiate iii the mixing chamber aqueous depth of about equal to the depth of the elec- The mixing chamber is heiihaiiy located immediately trodes beneath it. Chlorine produced at the anode and above the cell, but alternatively it may be apart from the caustic produced at h h d passed i th mixing cell. The pH in the mixing is preferably controlled at chamber by means f both natural fl d to h gas about heutrai or there sPeeiiieaiiy at about 6 to about evolution in the cell and the controlled feed to the anolyte 8. The most preferred reaction conditions are at a pH eompartmehh The pH in h i i h b was 0t to The PH in the hiixihg chamber is eehtieheti trolled near or at about neutral as indicated for each by Ohe or more of several methods one method is by example. The electrolyte from the mixing chamber was means of the how rate (it aheiyte iiquer to the anolyte passed to a reservoir having a liquid capacity of about compartment and the PH thei'eet- Another method is ten times that of the cell. The solution in the reservoir by controlling the amount of hydrochloric acid or chlorine was used as the cathoiyte feed liquor added to the anolyte feed electrolyte. By varying the addi- To remove sodium chiorate from th Ieservoir liquor tiehs 0t Hci or ehieiihe to the aheiyte the amount of a stream of liquor is continuously withdrawn from the chlorine produced for reaction with the caustic is changed, reservoir, passed through a erystaiiizer Wherein it iS thus atteetihg the PH iii the mixing chamber In ih chilled to reduce the solubility of sodium chlorate therethe how (it aheiyte u lesser amounts f the a i by precipitating a crop of sodium chlorate crystals. The iiqueh Pass ihte the mixing Chamber causing a rise in chlorate crystals are removed by filtration and the mother PH with the lesser how rates and a iewehhg of the PH liquor is subsequently returned to the anolyte compartwith increased flow rates. Also, secondary sources of acid meht of the cell as a replenished, acidified Sodium and/or caustic can be used for pH adjustments. chloride chlorate Solution The ieaetieii 0t hyiii'exyi ions with ehieiihe is ah In the cell, the anolyte compartment was enclosed with etheihue i'eaetieh- This favors the conversion 40 the exception of the electrolyte feed means and a series chlorite to chlorate. Therefore, the temperature in the of Small orifices in the top of the anolyte compartment mixing chamber is about the same to siightiy higher than as illustrated in the drawings, to provide for the upward ih the eeii; At h Preferred temperatures and e flow of anolyte liquor and the passage of the chlorine hyPeehieiite iapitiiy eehyeits to ehieiate- Seme addi' gas. The series of orifices substantially prevented the tiehai Period of time, ranging from about hve minutes downward flow of neutral electrolyte from the mixing to two hours is Preteiabiy Piei/itied te ietiuee the hype chamber to the anode compartment. The amount of HCl chlorite concentration as previously described. The aging added to the anolyte feed reduced the pH to less than is preferably effected in a container or reservoir either one, but provided a PH in the anolyte compartment of surrounding h eeii itseit from the about 3. The migration of hydroxy ions from the catho- The following examples 1llustrate certam preferred iyte compartment caused this increase i embodiments of the present invention. Unless otherwise In the Operation of the cell, the anolyte feed rate was indicated, all parts and percentages used herein and in 15 parts per minute and the eathoiyte feed was 100 parts the eiaiihs are by Weight and an temperatures are ih per minute. An electrolytic current was induced across giees Centigrade the cell through the bipolar electrodes by connecting the EXAMPLES l-6 terminal electrodes to a positive and negative source of An electrolytic cell is constructed in accordance with direct current The can temptirature the mixing chamher FIGS 1, 2 and 3 of the drawings using graphite anodes temperature and the reservolr temperature were mainand cathodes separated by a woven asbestos diaphragm. taiiied at about degrees eohtigrade' Table tabuiates The electrodes are about 3 incheS in length and about the results obtained with various current densiues, anolyte one inch thick. About of these electrodes are positioned 60 feed Concentrations and eatholyte feed Concentrationa di t f 0375 i h f h th i a concrete The current efliciencies were calculated based on cell gas container to form a cell. Sheets of glass insulate the analyses obtained by both Orsat and gas chromatographic electrodes from the container while forming watertight methods.

TABLE I Current Current Current Anolyte feed in grams Electrolyte out efiieiency efficiency density per liter (H01 is percent of mixing chamber based on based on gas (amperes by Weight) Catholyte feed iromreservoir in grams per liter (in grams perliter) Orsat chromatograper square analysis, phy analysis, inch) NaCl NaClO HCl pH NaOl NaClOa N32C1z07 NaClO NaClO pH percent percent 1 Not determined.

2 Control for Examples 3-5.

The above examples clearly illustrate that the present invention provides for the production of chlorates at extremely high efficiencies. The efiiciencies obtained are up to 10 percent higher than those obtained in conventionally used cells.

The process is also operated at higher temperatures with correspondingly good reults. With increased concentration of chlorate in the catholyte feed liquor, cell operating temperatures of about 80 degrees centigrade provide correspondingly good current efficiencies.

While there have been described various embodiments of the present invention, the apparatus and methods described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therein are possible. It is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner. It is intended to cover the invention broadly in whatever form its principles may be utilized.

What is claimed is:

1. A process for the production of an alkali metal chlorate which comprises:

(a) feeding an acidified solution of an alkali metal chloride to the anode compartment of an electrolytic cell having an anode and a cathode separated by a porous diaphragm to form an anode compartment and a cathode compartment;

(b) feeding a separate catholyte feed solution of an alkali metal chloride to said cathode compartment;

() imposing a decomposition voltage between said anode and said cathode thereby producing an anolyte solution containing chlorine and a catholyte solution containing an alkali metal hydroxide;

(d) passing said anolyte and said catholyte to a mixing ZOne wherein the chlorine in said anolyte and the alkali metal hydroxide in said catholyte react to pro duce a solution containing an alkali metal chlorate; and

(e) passing said solution containing said alkali metal chlorate to a reservoir from which said catholyte feed solution is drawn.

2. The process of claim 1 in which the solution fed to the anode compartment of said electrolytic cell is acidified with a compound selected from the group consisting of chlorine and HCl to obtain a pH of about one to four in the anode compartment.

3. The process of claim 1 in which the pH of the solu tion in said mixing zone is maintained in the range of 6 to 8.

4. The process of claim 1 in which the electrolytic cell is operated at a temperature of to 100 degrees centigrade.

5. The process of claim 1 in which the solution fed to said anode compartment contains alkali metal chlorate.

6. The process of claim 1 in which the feed rate to the cathode compartment is greater than the feed rate to the anode compartment.

7. The process of claim 1 in which said alkali metal chlorate is sodium chlorate.

8. The process of claim 1 in which said electrolytic cell is disposed in said reservoir and said catholyte feed solution is drawn from said reservoir by the evolution of hydrogen gas at the cathode.

References Cited UNITED STATES PATENTS 1,102,209 6/1914 Byrnes 204-98 2,829,095 4/1958 Oda et a1 20498 3,364,127 l/l968 Inoue et al. 20498 3,390,065 6/1968 Cooper 204- TA-HSUNG TUNG, Primary Examiner U.S. Cl. X.R. 204-98, 255 

